This invention relates to a magnetic recording medium, more specifically to a magnetic recording medium for high-density recording which is low in noise and is excellent in orientation properties.
Heretofore, most of the magnetic recording media have each been prepared by applying a magnetic coating composition composed of a magnetic powder and a resinous binder on a support substrate such as a polyester film. As the magnetic powders, there have mainly been used needle ferromagnetic powders comprising .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, their cobalt adsorption systems and CrO.sub.2. However, these needle ferromagnetic powders are small in their coercive force, and thus their recording density has already reached the limit. Therefore, these powders cannot satisfy recent requirements for high-density recording any more. For the purpose of overcoming this drawback, some attempts have been made to lessen a grain length of the needle ferromagnetic powder to the utmost or to improve a coercive force (Hc) and a maximum magnetic flux density (Ms). However, when the grain length of the needle ferromagnetic powder is reduced, electromagnetic conversion properties of the powder will be worsened in a low section by the influence of a reverse magnetic field, and thus a sufficient improvement in the needle ferromagnetic powder cannot be expected.
On the other hand, a so-called metallic tape is known in which the support substrate is coated with the magnetic coating composition prepared by dispersing the fine magnetic iron powder into the resinous binder. Although this metallic tape has the advantage that the magnetic recording can be carried out in a higher density than by the use of a conventional .gamma.-Fe.sub.2 O.sub.3 magnetic iron oxide material, it will easily be oxidized by oxygen in air, so that during the manufacturing process of the magnetic iron powder or the magnetic coating composition, the probability of a dangerous explosion will be extremely high, and its handling will be difficult. In addition thereto, such a metallic tape has the drawback that a stability which is one of the essential properties of the magnetic recording medium cannot be kept up for a long period of time.
As the magnetic powder by which the above-mentioned problems can be solved and the high-density recording can be accomplished, much attention is now paid to a hexagonal crystalline system ferromagnetic powder which is chemically stable and is suitable for a vertical magnetic recording. This vertical magnetic recording technique is essentially suitable for the high-density recording, since the higher the recording density becomes, the less the reverse magnetic field in the recording medium becomes.
The hexagonal crystalline system ferromagnetic powder generally comprises hexagonal plate-like grains and has magnetization-easy axis extending vertically to the plate surfaces of the grains. For this reason, even when coating is only made with these magnetic grains, the surfaces of the grains can easily be arranged in parallel with the support, therefore the magnetization-easy axis can be vertically oriented without any difficulty by means of a magnetic field orientation treatment or a mechanical orientation threatment. Hence, if the hexagonal crystalline system ferromagnetic powder having such properties is uniformly applied, as a fine single domain structure powder (average grain diameter 0.2 .mu.m or less), on the surface of the support together with the resinous binder, it will be expected to prepare the magnetic recording medium having an extremely high recording density.
However, when the average grain diameter of the hexagonal crystalline system ferromagnetic powder is 0.2 .mu.m or less, a cohesive force between the grains will be great, and it will thus be difficult to sufficiently disperse such a powder into the magnetic coating composition. As a result, the obtained magnetic recording medium will be poor in vertical orientation, will not be so good as expected at first in point of reproduction output properties, and will be poor in noise reduction properties.
Also when the average grain diameter of the magnetic powder is 0.2 .mu.m or more, such problems as mentioned above are likewise raised, though its dispersion properties can be improved a little.
Accordingly, it has been desired to improve the dispersion properties of the hexagonal crystalline system ferromagnetic powder and to build up, by the use of the improved ferromagnetic powder, the orientation properties and the noise reduction properties of the magnetic recording medium.